Violent protests against UN peacekeepers and Haiti's government are disrupting efforts to control the cholera epidemic, with fears that unrest will worsen in the runup to next week's election.

Riots in the north of the country spread to Port-au-Prince on Thursday, smothering swaths of the capital in choking black smoke from burning barricades.

UN staff were forbidden to venture out while crowds roved the streets tearing down election posters, erecting roadblocks and demanding the withdrawal of foreign peacekeepers who are suspected of bringing a disease that has killed 1,100 and infected more than 20,000 since last month.

Helicopters clattered over Port-au-Prince today monitoring the city for fresh flareups. Trucks and armoured vehicles with blue-helmeted troops, as well as Haitian police with shields and helmets, patrolled the streets.

"This is insane, how are we supposed to contain a cholera outbreak like this?" said one UN administrator.

In Cap-Haïtien, a northern city where the riots started earlier this week, people who were vomiting and had diarrhoea were unable to reach treatment clinics. Local media reported that bodies of those who succumbed were left on the streets.

"If the country explodes in violence then we will not be able to reach the people we need to," said Julie Schindall, an Oxfam spokeswoman.

Haitians believe peacekeepers from Nepal, which has cholera, flushed contaminated faeces from base latrines into the Artibonite river, triggering the outbreak. DNA fingerprinting has confirmed it is a south Asian strain which came from a single source. Travellers from Haiti brought cholera to the neighbouring Dominican Republic and Florida this week but authorities said controls and good sanitation should contain the disease. Ten inmates died in the overcrowded national prison in Port-au-Prince, raising fears for the health of 2,000 other inmates.

The outbreak of a disease unknown to Haiti for decades snapped the patience of those who consider the UN's peacekeeping mission, known as Minustah, a foreign occupier and enforcer for an unpopular government. Protesters held up signs saying "Minustah = Kolera" and smashed several UN vehicles. "Go back to your country because you bring cholera over here," one youth, his face shrouded in a T-shirt against tear-gas, shouted on Avenue La Lue, amid chanting, bottle-throwing companions. "We can't take it no more. Go back home."

A crowd threw rocks at a motorcade leaving the national palace, prompting a guard to lean out and fire warning shots to clear a path.

Rumours that the UN was poised to make an announcement about an investigation into the cholera source prompted a security alert by NGOs, who predicted worse mayhem should the 12,000-strong peacekeeping force be confirmed as the origin. The UN has admitted sanitation problems at the Nepalese base but said the soldiers tested negative for cholera. It blamed the violence on agitators trying to destabilise the run-up to next Sunday's presidential and legislative elections.

Doubts are growing over whether the vote will go ahead on schedule. Cholera is not transmitted through the air nor by touch but many Haitians, fearful and uncertain about an unfamiliar disease, may be reluctant to queue at crowded polling stations. There are also doubts over the election's legitimacy, given the provisional electoral council's perceived bias towards President René Préval and his favoured candidates. Candidates loyal to his ousted predecessor, Jean-Bertrand Aristide, were barred from running.

The aid agency Médecins sans Frontières today warned that "critical shortfalls" in the response to the epidemic were letting the crisis worsen.

"Despite the huge presence of international organisations in Haiti, the cholera response has to date been inadequate in meeting the needs of the population," said Stefano Zannini, the group's head of mission in Haiti.

"MSF is calling on all groups and agencies present in Haiti to step up the size and speed of their efforts to ensure an effective response to the needs of people at risk of cholera infection."

In a thinly-veiled swipe at other agencies and the UN it said more organisations were needed to treat the sick and prevent further infections. "There is no time left for meetings and debate. The time for action is now."